Thank you for the information.
But are those voices from the mentioned website still compatible with MeeGo's app?

Haven't got around to trying upon an N9 but whilst folder content is almost identical to the 4.5MB N9 voice file, there are an extra 10 items g5num_pt_1 to g5num_pt9 inclusive, g5num_0_75_mile and g5war_speed_limit_soft4edit9 compared to 4.2MB Symbian folder.

Whether this has any noticeable impact upon performance I know not.

As a post script have now discovered that Drive Voices for HERE WeGo downloaded upon Sailfish device contain for example the same english_female folder within, which was used upon N9 with only difference being the addition of an g5mod_003 file.